This invention relates generally to a method for improving the crystallinity of semiconductor ribbons, and more specifically to a method for simultaneously recrystallizing a plurality of semiconductor ribbons.
In the manufacture of semiconductor devices such as transistors, integrated circuits, photovoltaic devices and the like, the semiconductor industry uses large quantities of semiconductor material, usually silicon, in the form of thin wafers or sheets. It has been conventional to produce the semiconductor wafers by first growing a single crystal semiconductor ingot, sawing the ingot into a plurality of thin slices, and then lapping and polishing the slices to the desired thickness and surface finish. Although this process has proved satisfactory for most semiconductor devices, it is too expensive for some large area semiconductor devices and especially for large area photovoltaic devices or solar cells. In fact, in order that photovoltaic devices become a viable alternate energy source, a significant reduction in the cost of the semiconductor starting material is essential.
One technique which has been proposed and developed for the production of thin sheets of semiconductor material suitable for the production of solar cells is the so-called ribbon-to-ribbon (RTR) conversion process. ln this process a polycrystalline ribbon is transformed directly into a macrocrystalline ribbon without the need for costly processing of large diameter ingots. The RTR process uses one or more scanned beams of energy impinging upon one or both sides of a polycrystalline ribbon to locally melt the ribbon and to induce crystal growth as the ribbon is translated past the energy beam. As the molten zone moves along the ribbon, the material behind the zone resolidifies in a macrocrystalline form. The macrocrystalline structure is one in which the crystals are oi suificiently large size to permit efficient semiconductor action. Therefore, a monocrystalline ribbon wherein the ribbon is but a single crystal is encompassed within the term "macrocrystalline." In this context the word "ribbon" generally implies an elongate strip or sheet having a width much greater than its thickness. Typical dimension are a length of 15-30 centimeters, a width of 1-10 centimeters and a thickness of 50-250 micrometers.
The RTR conversion technique has proved successful in producing semiconductor substrates at a reduced cost for use in fabricating photovoltaic devices. It is desirable, however, to even further reduce the cost of the conversion process, for example, by reducing the amount of energy required to accomplish the melting. It is further desirable to alter the thermal profiles experienced by the ribbon during conversion to thereby improve the quality of the converted material.
It is therefore an object of this invention to provide an improved process for the conversion of polycrystalline material to macrocrystalline material.
It is another object of this invention to provide an improved RTR process having an increased throughput and a reduced energy cost.
It is a still further object of this invention to provide an improved RTR process for producing silicon ribbons of improved crystal quality.